1. Field of the Invention
The present invention relates to a nonmagnetic one-component toner, a kind of toner for electrophotography used for development of electrostatic latent images in electrophotography, electrostatic recordings, and electrostatic printing.
2. Discussion of the Related Art
As disclosed in U.S. Pat. Nos. 2,221,776, 2,297,691 and 2,357,809 and other publications, conventional electrophotography utilized in apparatuses for forming fixed images, such as laser printers and dry-type electrostatic copy machines, comprises the steps of forming an electrostatic latent image by evenly charging a photoconductive insulating layer (a charging process) and subsequently exposing the layer to eliminate the charge on the exposed portion (an exposing process) and visualizing the formed image by adhering colored charged fine powder known as a toner to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process). This electrophotography may further comprise, subsequent to the transferring of the visible image, scraping off residual toner on the photoconductor for the purpose of cleaning the photoconductor surface (a cleaning process).
In the developing process of the above electrophographic methods, there have been proposed as the most convenient method two-component magnetic brush developing methods using a developer consisting of two components, namely, a toner and a carrier, the carrier being used for the purposes of supplying electric charges to the toner and for conveying the charged toner onto the latent image portion by a magnetic force.
However, in the two-component magnetic brush developing method, since a magnetic force is utilized in the conveying of the developer, a magnet has to be placed in the developer roller, and the carrier is made of a metal or an oxide thereof such as iron powder, nickel powder, and ferrite. Therefore, the developer device and the developer become undesirably heavy, thereby making it difficult to miniaturize and thus reduce the weight of the overall recording device.
On the other hand, as disclosed in U.S. Pat. Nos. 3,909,258 and 4,121,931, there have been conventional well used magnetic one-component developing methods comprising the step of conveying a toner to the latent image portion without using a carrier, the methods being carried out by utilizing a magnetic force owned by the toner containing a magnetic substance therein. However, a magnet has to be also used in the inner portion of the developer roll in this developing method, making it difficult to reduce the weight of the developer device.
In order to solve the problems in these developing methods, much studies have been recently conducted on nonmagnetic one-component developing methods wherein a toner alone is used without containing any magnetic powder, as disclosed, for instance, in U.S. Pat. Nos. 2,895,847 and 3,152,012, and Japanese Patent Examined Publication Nos. 41-9475, 45-2877 and 54-3624.
However, in the conventional nonmagnetic one-component developing methods, since toners are provided with electric charges only at an instant when the toner passes near the charging blade, the charging control of the toner in these methods is extremely difficult. In order to solve this problem, there have been proposed a method in which a silica fine powder subjected to a surface-treatment with a titanate coupling agent is added to the surface of the toners (see, for instance, Japanese Patent Laid-Open No. 59-231549). On the other hand, it is also important to improve the contact efficiency of the toner with the charging blade. In order to achieve good contact efficiency, various external additives have been studied, as disclosed, for instance, in Japanese Patent Laid-Open Nos. 64-77075, 3-294864, etc. Further, for the purpose of solving the problem of poor charging of small toners passing besides large toners, a particle size distribution has been also studied as disclosed, for instance, in Japanese Patent Laid-Open No. 63-279261.
However, the above developing methods alone do not provide sufficient charge control, and particularly there are problems in nonmagnetic one-component developing, so that improvements in background level cannot be sufficiently found.
Also, in order to keep the charges on the toner, the triboelectric properties owned by a resin constituting the main component of a toner can be utilized. However, the triboelectric properties of the toner are small in this method, so that the background level of the visible images is liable to be increased during development, and thereby unclear fixed images are liable to be formed. Therefore, in order to give desired triboelectric properties to the resulting toners, addition of dyes or pigments and further, charge control agents, has been proposed for the purpose of controlling triboelectric properties.
However, in the above methods, although toners may provide good fixed images in the beginning, since the developer roller and toner carriers such as carriers are liable to be contaminated during repeated copying and supplying of the toner, such problems arise, that the background level increases and that the image density decreases. One type charge control agent has low triboelectric charges and is more liable to be affected by environmental conditions such as temperature and humidity, so that changes in environmental conditions cause changes in image density in the resulting toners. Another type of charge control agent has poor dispersibility in the resin, so that the triboelectric charges between the particles in the resulting toner are likely to be uneven, thereby making it liable to cause increase in the background level. Still another types of charge control agent has poor storage stability, so that the triboelectric charges of the resulting toner are liable to be lowered during long-term storage.
In order to solve these problems, Japanese Patent Examined Publication Nos. 43-17955, 55-42752, and 53-1994 disclose the use of various metal complexes as negative charge control agents. Although these charge control agents certainly show excellent triboelectric charges, these compounds are chromium-based, and therefore much improvement is desired from the aspect of safety.
In view of the above, Japanese Patent Laid-Open Nos. 61-155464, 61-101558, and 61-155463 disclose the use of iron complexes (iron compounds) as charge control agents. These publications disclose that the iron complexes give excellent negative chargeability in the resulting toner and have excellent compatibility with the resin. However, these publications do not specifically disclose a toner obtainable by using a polyester, as a main component, as mentioned below, and mere use of these iron complexes for a nonmagnetic one-component toner does not altogether solve the problems concerning the background levels mentioned above for the reasons given below. The nonmagnetic one-component developing method is completely different from other kinds of developing methods in the steps of conveying and charging the toner. Also, depending upon the binder resins used, the compatibility of the iron complexes with the binder resin and the triboelectric properties in the nonmagnetic one-component developing method notably change. Therefore, it has been difficult to select a resin suitable for a nonmagnetic one-component toner and types and amounts of iron complexes.
On the other hand, as for binder resin for toners, various resins, including styrenic copolymers, such as polystyrenes, styrene-butadiene copolymers, and styrene-acrylic acid copolymers; ethylenic copolymers, such as polyethylenes and ethylene-vinyl acetate copolymers; poly(meth)acrylic acid esters; polyester resins; epoxy resins; and polyamide resins, have been used. Among these resins, the polyester resins are particularly used as resins for toners having excellent low-temperature fixing ability. Also, the polyester resins inherently have good resin toughness, so that the durability of the resin can be improved while retaining the low-temperature fixing ability, and thus making them suitable for nonmagnetic one-component toner wherein a stress is more likely to be exerted to a toner by a charging blade.
Therefore, particularly in a nonmagnetic one-component toner using polyester resins, the development of a toner with sufficient charge control in order to make the background level low is in demand in the art.